We examine the behavior of turn-final <i>but </i>in a corpus of spoken American and Australian English, proposing two hypotheses. First, the behavior of <i>but </i>can be modeled as a continuum from a prosodic-unit-initial to a prosodicunit-final discourse particle. Second, as <i>but </i>“moves” along this continuum, its conversational function changes, in a way that is consistent with what has been described in the grammaticization literature.In both our American and Australian data, both prosodically and sequentially, speakers give evidence of taking another’s prior <i>but</i>-ending utterance as having been finished, but with an implication left “hanging”. However, our Australian data provide considerable evidence of Australian English “final <i>but</i>” having become a “fully-developed” final particle marking contrastive content.